1. Field of the Invention
This invention relates generally to drill strings for drilling directional wellbores and more particularly to a self-adjusting steerable drilling system and method for drilling directional wellbores.
2. Description of the Related Art
Steerable motors comprising a drilling or mud motor with a fixed bend in a housing thereof that creates a side force on the drill bit and one or more stabilizers to position and guide the drill bit in the borehole are generally considered to be the first systems to allow predicable directional drilling. However, the compound drilling path is sometimes not smooth enough to avoid problems with the completion of the well. Also, rotating the bent assembly produces an undulated well with changing diameter, which can lead to a rough well profile and hole spiraling which subsequently might require time consuming reaming operations. Another limitation with the steerable motors is the need to stop rotation for the directional drilling section of the wellbore, which can result in poor hole cleaning and a higher equivalent circulating density at the wellbore bottom. Also, this increases the frictional forces which makes it more difficult to move the drill bit forward or downhole. It also makes the control of the tool face orientation of the motor more difficult.
The above-noted problems with the steerable drilling motor assemblies lead to the development of so called xe2x80x9cself-controlledxe2x80x9d or drilling systems. Such systems generally have some capability to follow a planned or predetermined drilling path and to correct for deviations from the planned path. Such self-controlled system are briefly described below. Such systems, however, enable faster, and to varying degree, a more direct and tailored response to potential deviation for directional drilling. Such systems can change the directional behavior downhole, which reduces the dog leg severity .
The so called xe2x80x9cstraight hole drilling devicexe2x80x9d (xe2x80x9cSDDxe2x80x9d) is often used in drilling vertical holes. An SDD typically includes a straight drilling motor with a plurality of steering ribs, usually two opposite ribs each in orthogonal planes on a bearing assembly near the drill bit. Deviations from the vertical are measured by two orthogonally mounted inclination sensors. Either one or two ribs are actuated to direct the drill bit back onto the vertical course. Valves and electronics to control the actuation of the ribs are usually mounted above the drilling motor. Mud pulse or other telemetry systems are used to transmit inclination signals to the surface. The lateral deviation of boreholes from the planned course (radial displacement) achieved with such SDD systems has been nearly two orders of magnitude smaller than with the conventional assemblies. SDD systems have been used to form narrow cluster boreholes and because less tortuous boreholes are drilled by such a system, it reduces or eliminates the reaming requirements.
In the SDD systems, the drill string is not rotated, which significantly reduces the hole breakout. The advantage of drilling vertical holes with SDD systems include: (a) a less tortuous well profile; (b) less torque and drag; (c) a higher rate of penetration; (d) less material (such as fluid) consumption; (e) less environmental impact; (f) a reduced risk of stuck pipe; (g) less casing wear, and (h) less wear and damage to drilling tubulars.
An automated drilling system developed by Baker Hughes Incorporated, the assignee of this application, includes three hydraulically-operated stabilizer ribs mounted on a non-rotating sleeve close to the drill bit. The forces applied to the individual ribs are individually controlled creating a force vector. The amount and direction of the side force are kept constant independent of a potential undesired rotation of.the carrier sleeve. The force vector can be pre-programmed before running into the borehole or changed during the drilling process with commands from the surface.
This system has two basic modes of operation: (i) steer mode and (ii) hold mode. In the steer mode the steering force vector is preprogrammed or reset from the surface, thus allowing to navigate the well path. In the xe2x80x9chold modexe2x80x9d values for inclination and/or azimuth are preset or adjusted via surface-to-downhole communications, thus allowing changes to the borehole direction until the target values are achieved and then keeping the well on the target course. As the amount of side force is preset, the turn radius or the equivalent build-up rate (BUR) can be smoothly adjusted to the requirements from 0 to the maximum value of 8xc2x0/100 feet for such a system.
An automated directional drilling bottomhole assembly developed by Baker Hughes Incorporated and referred to as AUTOTRAK has integrated formation evaluation sensors to not only allow steering to solely directional parameters, but to also take reservoir changes into account and to guide the drill bit accordingly. AUTOTRAK may be used with or without a drilling motor. Using a motor to drive the entire assembly allows a broader selection of bits and maximizes the power to the bit. With a motor application, the string rpm becomes an independent parameter. It can be optimized for sufficient hole cleaning, the least casing wear and to minimize dynamics and vibrations of the BHA, which heavily depend on the rotational string frequency.
One of the more recent development of an automated drilling system is an assembly for directional drilling on coiled tubing. This system combines several features of the SDD and the AUTOTRAK system for coiled tubing applications. This coiled tubing system allows drilling of a well path in three dimensions with the capability of a downhole adjustable BUR. The steering ribs are integrated into the bearing assembly of the drilling motor. Other steering features have been adopted from the AUTOTRAK with the exception that the steering control loop is closed via the surface rather than downhole. The fast bi-directional communication via the cable inside the coil provides new opportunities for the execution of well path corrections. With the high computing power available at the surface, formation evaluation measurements can be faster processed and converted into a geosteering information and imported into the software for the optimization of directional drilling.
A coiled tubing automated drilling system is disclosed in the U.S. Ser. No. 09/015,848, assigned to the assignee of this application, the disclosure of which is incorporated herein by reference.
The steering-while-rotating drilling systems can be further enhanced through a closed loop geosteering by using the formation evaluation measurements to directly correct the deviations of the course from the planned path. A true navigation can become possible with the integration of gyro systems that withstand drilling conditions and provide the required accuracy. With further automation, the manual intervention can be reduced or totally eliminated, leaving the need to only supervise the drilling process. Both supervision and any necessary intervention can then be done from remote locations via telephone lines or satellite communication.
The trend in the oil and gas industry is to drill extended reach wells having complex well profiles. Such boreholes may have an upper vertical section extending from the surface to a predetermined depth and one or more portions thereafter which may include combinations of curved and straight sections. For efficient and proper hole forming, it is important to utilize a drill string that has full 3-D steering capability for curved sections and is also able to drill straight sections fast which are not rough or spiraled.
The present invention addresses the above-noted problems and provides a drilling system that is more effective than the currently available or known systems for drilling a variety of directional wellbores.
The present invention provides a drilling system for drilling deviated wellbores. The drilling assembly of the system contains a drill bit at the lower end of the drilling assembly. A motor provides the rotary power to the drill bit. A bearing assembly disposed between the motor and the drill bit provides lateral and axial support to the drill shaft connected to the drill bit. A steering device provides directional control during the drilling of the wellbores. The steering device contains a plurality of ribs disposed at an outer surface of the drilling assembly. Each rib is independently controlled and moves between a normal or collapsed position and a radially extended position. Each rib may exert force on the wellbore interior when urged against the wellbore. Power units to independently control the rib actions are disposed in the drilling assembly. A controller carried by the drilling assembly controls the operation of the power units in response to directional and navigational sensors in the drilling assembly. Sensors to determine the amount of the force applied by each rib on the wellbore may be provided. A second set of ribs axially spaced apart from the first set, is preferably provided. This allows the drilling of a greater range of curved holes and better control over straight hole drilling.
The curved holes are drilled by rotating the drill bit by the mud motor and by independently adjusting the rib forces. The drill string is kept stationary. Vertical sections are drilled in a similar way. To compensate for a deviation from the vertical, selected forces can be individually applied to the ribs in order to generate a force vector in the plane orthogonal to the borehole axis. It is also possible to apply the same force or no force to the ribs and even rotate the drill string. Straight inclined sections can be drilled without string rotation with a proper force adjustment on the steering ribs to accomplish straight drilling. To reduce the friction while longitudinally moving the drilling assembly, to improve the hole cleaning and the cuttings transport, and to deliver more power to the bit, the drill string can be continuously rotated at any speed required while drilling straight inclined sections. To control the drilling direction in the vertical plane while rotating the string, the same force is applied to all of the ribs. The magnitude of this force is selected such that the required directional tendency is achieved.
Force vectors or the magnitude of the forces are adjusted if the drilling direction differs from the defined course. The system is self-adjusting and operates in a closed loop manner. Inclination and navigation sensor data is processed by a downhole controller. The force vectors may be programmed in the downhole controller. Command signals from a surface controller may be sent to initiate the setting and/or adjustment of the rib force vectors in accordance with the planned wellbore course (path).
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.